Ink jet printing is a non-impact printing process in which the printer produces droplets of ink in response to digital signals, such as those generated by a computer. The droplets of ink are deposited on a substrate such as paper or transparent films. Ink jet printers have found broad commercial acceptance due to their print quality, low cost, relatively quiet operation, and graphics capability.
The inks used in ink jet printers can be classified as either dye-based inks or pigment-based inks. Dye-based inks are satisfactory for most applications, but generally have poor light fastness and water resistance. As a printed document is expected to have a certain degree of permanency, the lack of light fastness and water resistance of the printed image derived from dye-based inks is a problem. Pigment-based inks can be prepared which have excellent light fastness and water resistance. Thus, for purposes of obtaining a printed document with a reasonable degree of permanency, pigment-based inks are preferred over dye-based inks.
A great concern with ink jet printing is the level of print quality, as defined by edge acuity or sharpness of an image and minimal feathering, which can be obtained on “plain paper.” In recent years there has been an increasing demand for ink jet printers that provide excellent print quality on plain paper. The present invention is mainly concerned with print quality as defined by edge acuity or sharpness of the printed images on plain paper. In terms of print quality on plain paper, suitably designed pigment-based inks are especially desired.
When a liquid ink droplet contacts the paper surface as a result of ink jet printing, the liquid spreads out from the impact origin and penetrates the paper. Cellulose fibers, present in most plain papers, tend to act as wicks that draw the liquid along the length of the individual fibers by capillary action. In dye-based inks, in which the colorant is homogeneously dissolved in the liquid, the colorant will spread out, penetrate, and be drawn along the length of cellulose fibers to the exact same degree as the liquid. The typical result for a dye-based ink is a colored dot that has poorly defined feathered edges.
In pigment-based inks, in which the colorant is homogeneously dispersed in the liquid, unless the dispersion stability of the colorant is disrupted upon contact with the paper, the colorant will spread out, penetrate, and be drawn along the length of cellulose fibers to the nearly the same degree as the liquid. The typical result for a conventional pigment-based ink is a colored dot that has poorly defined feathered edges.
In contrast, a suitably designed pigment-based ink, in which the dispersion stability of the colorant is disrupted upon contact with the paper, the colorant will not spread out, penetrate, and be drawn along the length of cellulose fibers in the same way as the liquid. For this type of ink, the colorant effectively separates from the liquid carrier. The result is a colored dot that has a sharp edge boundary with negligible feathering.
Water-based pigment dispersants are well known in the art, and have been used commercially for applying films, such as paints, to various substrates. A dispersant using a polyuronic acid is also suggested. For example, in U.S. Pat. No. 6,242,529, a polyuronic acid derivative in which a hydrophobic polymer is covalently attached to the reducing terminus of the polyuronic acid is disclosed as a dispersant. These hydrophobic polymers include photopolymers or copolymers prepared from at least one monomer selected from the group consisting of styrene or substituted styrenes, vinyl pyridine or substituted pyridines, methacrylic acid esters, acrylic acid esters, acrylonitrile, methacrylonitrile, butadiene, and isoprene. The hydrophobic polymers also include poly(dimethylsiloxane), hydrophobic polyamides, and hydrophobic polyamines.
On the other hand, one general approach for obtaining reliable aqueous pigment-based inks is to use self-dispersed pigments in the ink formulations. As the descriptive phrase, “self-dispersed,” is commonly used, these pigments do not require dispersing agents, such as polymeric dispersants or surfactants to produce stable dispersions of the pigment in the aqueous vehicle. The means by which the pigments are self-dispersed is the deliberate introduction onto the surface of the pigment particles of a sufficient number of charged functionalities. This approach has been widely applied to black pigments derived from carbon black.
With regard to print quality on plain paper, aqueous pigment-based inks that use self-dispersed pigments in the ink formulations are advantageous. Specifically, the print-quality advantage arises from the fact that inks can be formulated with relatively high pigment contents when self-dispersed pigments are used. High pigment contents generally translate into high optical densities when the inks are printed on plain paper. As noted above, high optical densities typically result in images that are preferred by consumers in comparison with images with lower optical densities that are perceived as “dull.”
When printing by using aqueous pigment-based inks comprising self-dispersed pigments, however, they yield images with exceptionally poor lustrousness and exceptionally poor adhesion on coated specialty media. These failings are generally attributed to the non-inclusion of polymeric dispersing agents in the ink. Polymeric dispersing agents, which typically are resinous and partially bonded to the pigment surfaces, act as both a glaze for smoothing out the rough surface of the pigment and as a binder for mediating adhesion between pigment particles and adhesion between pigment particles and the surface of the specialty media.
A simple and obvious approach to overcoming the adhesion deficiency of inks that use self-dispersed pigments is to add a resinous polymeric binder to the ink composition. The inclusion of the resinous binder, which can function as a glaze, also improves the lustrousness of the printed images on coated specialty media in comparison to those images derived from unadulterated self-dispersed pigment formulations. As far as the present inventers know, the self-dispersed pigment/binder combinations may fall considerably short of the adhesion and lustrousness that is achieved routinely using non-self-dispersed pigment dispersions, in which a dispersing agent is required to produce a stable dispersion.
There is a demand for pigment dispersed aqueous ink compositions that give reliable printing performance and yield is printed images having excellent print quality.
Further, there remains a demand for self-dispersed pigment aqueous ink compositions that give reliable printing performance and yield excellent print quality, specifically on plain paper.
Furthermore, there is a demand for self-dispersed pigment aqueous ink compositions that yield excellent print quality on coated specialty media that exhibit lustrousness. In particular, there remains a demand for self-dispersed pigment aqueous ink compositions that yield print quality on coated specialty media such that good lustrousness and good adhesion are obtained.